Light Emitting Diodes (LEDs) are semiconductor diodes made of special material that radiate visible and invisible light (covering infrared to ultraviolet range of electromagnetic spectrum) upon spontaneous recombination of electrons and holes in the p-n junction. A forward biased voltage is normally applied to the p-n junction to accelerate the electron-hole recombination and produce sufficient brightness. The wavelength (and therefore color) of the emitted light depends on the energy of the semiconductor band gap. Early LEDs emitted low intensity red lights. Only recently have new semiconductor materials been available with larger energy gaps allowing LEDs to emit green and later on blue lights. Further, advancements in increasing the brightness and efficiency of LEDs have led to the invention of white LEDs.
A liquid crystal display (LCD), commonly used for TV panels and computer monitors, utilizes the light modulating properties of liquid crystals (LCs). LCs are transmissive elements. They merely guide and do not emit light directly. Therefore, LCD panels produce no light of their own and require an external lighting mechanism to be visible. Conventionally, a Cold Cathode Fluorescent Lamp (CCFL) situated behind the LCD panel, have provided the lighting. More recently, with advancements in HD TV and higher frequency video contents, LED backlit LCD panels have appeared in the television industry as an alternative to CCFL backlit LCDs. There are two kinds of LED backlighting techniques, white LED backlighting and Red/Green/blue (RGB) LED backlighting. A white LED (widely used in notebooks and desktop screens) is actually a blue LED combined with yellow phosphor to give the impression of white light. The spectral curve has big gaps in the green and red parts, in this case. RGB LEDs consist of a red, a green, and a blue LED and can be controlled to produce different temperatures of white. RGB LEDs can deliver an enormous color gamut to screens. The backlight from three separate LEDs can produce a color spectrum that closely matches the color filters in the LCD pixels themselves. This way, the LCD color filter pass-band can be narrowed so that each color component lets only a very narrow band of spectrum through the LCD. That improves the power efficiency of the display since a minimal amount of light is blocked when white is displayed. Also, the actual red, green, and blue points can be moved farther out so that the display is capable of reproducing more vivid colors. Both types of LED backlights can be arranged in arrays to light a screen.
LEDs present many advantages over incandescent light sources including larger color gamut, higher luminous efficiency, deeper black level (higher contrast), lower energy consumption (reduced waste light), longer lifetime, improved robustness, smaller size, faster switching response, and greater durability and reliability. In view of the demands for lower cost and power, lower environmental impact (green) and thinner displays, the industry has undertaken a major effort of improving the backlight technology by a rapid transition from the older CCFL backlighting to the more efficient and flexible LED backlighting. The U.S. Pat. No. 6,888,529 discloses an example of such system wherein an array or banks of RGB LEDs are driven by special circuitries to deliver light to each pixel of the display. The intensity and color content can be adjusted at source by manipulating individual color components directly through the driving circuits.
There exist however several drawbacks of LED sources. The most major drawback is increased color and brightness non-uniformity, due to the fact that LEDs are discrete light sources. Because of the variation in the manufacturing of LEDs and in LED aging, with each LED aging at a different rate, the uniformity is significantly decreased in relation to CCFL backlighting. A wide screen TV has a large number of LEDs (order of 100s) that are needed for a display in comparison with small number of CCFL tubes (order of 10s). Each individual LED has a different level of brightness. Even with sorting and binning, these light sources can have as much as +/−10% variation in brightness, device to device. Further, the use of three separate light sources for red, green, and blue means that the white point of the display can move as the LEDs age at different rates. Aging also occurs with White LEDs, with changes of several 100K of color temperature. White LEDs may suffer from blue shifts at higher temperatures too. As a result, they require more precise current and heat management than traditional light sources, therefore more expensive to build. In fact, if a certain level of uniformity is not achieved, the display will be rejected after production causing loss for the manufacturer.
In recent years, Organic Light Emitting Diodes (OLED) have been used in TV screens and other displays as an alternative to LCDs. Unlike an LC element, an OLED is an active element wherein lies an emissive electroluminescent layer of organic semiconductor compounds which emit light in response to an electric current. This layer is a thin film situated between two electrodes, one of which is normally transparent. The organic compounds are either small molecules or polymers that allow OLEDs to be used directly as display pixels. Thus an OLED display functions without a backlight. In another word, the backlight and the modulator planes are the same.
In general, an image from a display, LCD, OLED or otherwise, is a spatial varying pattern of color and brightness, which aims to match the pattern of an input signal. If the input signal is spatially constant, then its reproduction by a display is expected to be constant in color and brightness. This is referred to as the requirement of color and brightness uniformity, which is a critical requirement for accurate color reproduction by a display. An LCD panel has several other components including light guides and diffusers meant for directing and evenly distributing the light towards the front. Although these elements help improve uniformity, their design becomes complicated with a decreasing panel thickness trend, in turn leading to reduced effectiveness. There is a need for alternative methods of color and brightness adjustments in order to address the abovementioned shortcomings in a more effective and affordable manner.
There exists some prior art solutions on improving the quality of the backlighting mainly by manipulating the source voltage after sensing or visualizing the output signal. These solutions address uniformity in view of primarily the backlight panel, and not the LC panels where the image is actually viewed, therefore have limited effectiveness. For example US 2007/0200513 discloses means for controlling LED drives in response to changes in temperature and voltages. US 2006/0007097 discloses a backlight adjustment method for an LED backlit LCD apparatus. Luminance measurement sensors are integrated with the LCD panel by being disposed on a substrate together with thin file devices as pixels in the panel. None of these addresses the non-uniformities as a viewer would observe. Further, no solution for adjusting the chromaticity of the signal has been provided.
It is an object of one or more aspects of the present invention to provide electronic means for improving color and brightness uniformity of LCD displays addressing simultaneous adjustment of both the backlight source and the LC the light modulators. All aspects of the invention related to the modulator are applicable regardless of the light source.
In one embodiment, LEDs provide the backlight. Not only is the non-uniformity due to aging LEDs addressed, but the present approach can help manufactures in cost savings by having fewer units and tolerating units that would otherwise have failed the thresholds and been discarded. The teachings are also applicable to traditional CCFL backlight sources consisting of a plurality of CCFL tubes.
In another embodiment, Laser Diodes (LDs) are used as the backlighting source. LDs work in a similar fashion as LEDs when used to backlight LCD panels. The principal difference is in light generation where the recombination of electrons and holes occurs as stimulated rather than spontaneous, which is of course a necessary condition for lasing in general. The wavelength spectra of LDs are much narrower than that of the LEDs and result in even more well-defined colors.
The light source components and related figures presented herein primarily illustrate a direct LED backlighting system as an exemplary embodiment. However, as far as the displayed light is concerned, as mentioned, the invention is applicable to edge-lit LEDs, LDs and CCFLs as well as OLEDs (OLEDs being a special case of direct LED).